The need for high intensity, spatially uniform illumination sources is particularly acute in optical metrology instrumentation such, for example, as interferometric microscopes. In the past, sources of filtered and unfiltered white light and arc sources, gas lasers, laser diodes, and light emitting diodes (LED's)--each for the most part lifted, without significant modification or effective adaptation, from standard optical microscopes--have been employed. However, the acquisition of accurate measurement data using interferometric microscopes requires illuminator properties that notably differ from those suitable for standard optical microscopes.
For example, phase shifting interferometry instruments are routinely used in precision automated quality and process control applications in which data is repeatedly taken without direct operator intervention; the illumination source must accordingly be extremely rugged and long lived. Moreover, interferometric data is generally obtained using image sensors and, since the illumination wavelengths should be selected to conform to the spectral sensitivity and resolution requirements of the sensor, the ability of the human eye to image or view the illumination is generally irrelevant to wavelength selection.
Depending upon sensor type, interferometric illuminators may require shuttering during data readout and should therefore be 100 percent modulatable at relatively high speeds since, in typical applications, multiple frames of data are taken in synchronization with a reference surface phase shifter. In addition, the interference fringe contrast, rather than feature contrast, should be maximized and the fringes must have negligible contrast variation over the range of reference surface axial motion; accordingly, the temporal coherence length of the illumination should be long as compared to the reference surface axial motion.
Further requirements of interferometric microscope illumination are that its mean wavelength be precisely known, that its spectral properties and intensity not vary (i.e. high temporal uniformity) as data is being taken, that it be sufficiently intense as to reduce data acquisition time and, correspondingly, sensitivity to vibration, that its source not be a direct or indirect generator of measurement-disturbing vibrations, that it be spatially incoherent so as to reduce optical artifact sensitivity and speckle, and that it be sufficiently uniform as to provide a well understood spatial coherence function. It is also preferred that the illuminator provide ready access to a multiplicity of different wavelengths of light for extending the instrument's step measurement size.
All heretofore known and used illumination sources for optical metrology instrumentation fail to meet these multiple concurrent requirements. For example, filtered white light and arc sources satisfy the coherence requirements and are simple sources of multiple illumination wavelengths. However, they are short lived, are physically large with much associated optics, require mechanical shuttering and wavelength selection systems, and generate undue amounts of heat.
Gas laser sources satisfy the wavelength precision and stability requirements and are cooler and longer lived than incandescent sources. Moreover, unlike filtered white light or arc sources, the long temporal coherence properties of gas lasers permit their use with non-equal path (i.e. FIZEAU) type interferometers. On the other hand, they suffer from the need to reduce their spatial coherence, as by associated mechanical rotating diffuser disks which are both physically large and a potential source of vibration, they require mechanical shuttering systems, and they have no intrinsic wavelength selection capability. In addition, laser moding from thermally-induced cavity variations can be a major source of intensity fluctuations which are difficult to remove.
Laser diodes satisfy the wavelength precision requirement, are easily modulated, and are cooler in operation than arc, incandescent and gas laser sources and much longer lived. Moreover, the temporal coherence properties of index guided solid state lasers is comparable to gas lasers, and the intensity of laser diodes is sufficiently constant if their temperature is stabilized and optical feedback is provided. Although they have no intrinsic wavelength selection, the small size of laser diodes permits the use of multiple diodes for multiple wavelengths; however, the wavelengths vary from diode to diode and not all wavelengths, particularly short wavelengths, are available. Furthermore, laser diodes are high static sensitive and therefore require special handling precautions and circuitry and, as with gas lasers, they also suffer from laser moding and the need to break the spatial coherence as, for example, through the use of mechanical rotating diffuser disks.
Light emitting diode sources can potentially provide the advantage of long life, high speed modulation capability and low cost. Illuminators using LED's for standard microscopes, as for example described in U.S. Pat. No. 4,852,985, are not simultaneously of sufficient uniformity and intensity to provide a cost-effective source for automated precision optical metrology instrumentation. Imaging of a two-dimensional array of light emitting diodes onto a diffuse surface in an interferometric measurement apparatus requires high numerical aperture optics for good collection efficiency and results in a large, relatively expensive source with a sinusoidally-varying intensity distribution on the diffuser surface on which the semiconductors are imaged. Improvements to uniformity can be realized only at the cost of a thicker, more diffuse surface--which unavoidably and undesirably reduces illumination intensity. In addition, temporal intensity variations due to common mode electrical power fluctuations not visible to the eye can completely destroy interferometric data.
There is, accordingly, an unmet need for an illuminator that is especially well-suited for and concurrently satisfies the various requirements of automated precision optical metrology instrumentation, particularly for interferometric microscopes.